1. Field of the Invention
The present invention relates to the field of high voltage vacuum switches and circuit interrupting devices and more particularly to a vacuum switch with a pre-insertion resistor or inductor arrangement to limit transient in-rush currents and or voltage transients during the closing and opening of a power distribution circuit containing capacitor banks.
2. Discussion of the Prior Art
A number of vacuum and non-vacuum prior art arrangements are directed to pre-insertion resistors or inductors for circuit interrupting devices wherein a resistor or inductor is either inserted in series with a high voltage switch or in parallel with a switch gap during the closing movement of the switch or interrupting unit to reduce audible and electrical noise and to limit transient in-rush current and/or voltages incident to completion of the circuit by the switch or interrupting unit. For example pre-insertion resistors of this type are shown in the following U.S. Pat. Nos. 3,588,406; 3,576,414; 3,566,061; 3,590,186; 3,763,340; 4,069,406; 4,072,836; 4,324,959; 4,695,918 and 4,788,390. Without the pre-insertion resistor, as the circuit interrupting device is closed, the in-rush current may reach values of 10 to 30 thousand amperes, where the interrupting device is used in conjunction with back to back capacitor banks. Additionally, during energization of a single capacitor bank, large voltage transients may also be produced. Such transient current and/or voltages can produce undesirable noise both audible and electrical and can, of course, also lead to distress or damage to equipment connected to the circuit. With the pre-insertion resistor, the in-rush current arising from switching back to back capacitor banks is limited to much lower values, perhaps in the range of 1.5 to 4 thousand amperes, which can be carried by the circuit without undue distress. Since the pre-insertion resistor or inductor is in the circuit only briefly during the closing of the circuit interrupting device, the pre-insertion resistor or inductor is not required to carry the continuous current of the circuit except during the portion of the insertion time after the in-rush. The vacuum devices of this type rely on complex and costly external switching techniques, while the non-vacuum devices rely on an air switch, which is quite noisy and bulky or SF6 devices, which are now creating environmental concerns due to the affect of escaped SF6 gas on the ozone layer.
Another approach to damping or limiting the current in-rush incident to the completion of the capacitor bank circuit by a high voltage switch is the continuous, permanent connection of an inductor in the circuit. However, such an arrangement does have its drawbacks since the inductor must be designed to carry continuous load currents and fault currents. In addition, there are ongoing costs associated with power losses in the inductor on a continuous basis as well as a reduction in the effectiveness of the capacitor bank to which it is connected.
Vacuum interrupters have been used in series combinations or with other circuit interrupting devices to provide a pre-insertion means. U.S. Pat. No. 3,708,638 illustrates two vacuum circuit breakers connected in series with an electronic control system to close one breaker before the other. This results in an arrangement that is complex and costly. U.S. Pat. No. 4,383,150 illustrates a vacuum interrupter combined with an SF6 interrupter. The combination of the two interrupters results in a switching device, which is also complex, costly and has the aforementioned environmental concerns associated with SF6 gas.
Prior art electronically controlled vacuum switches have allowed for precise closing on a voltage zero which minimizes the in-rush current and voltage transients as is illustrated in U.S. Pat. No. 6,921,989 B2. The electronic control employees a feedback circuit to determine the exact location and speed of the contact operating means so that the vacuum switch can be closed on a voltage zero of the sinusoidal waveform of the electric supply line. This type of vacuum switch is quite complex and costly, and can be difficult to set up when utilized in three phase applications.
Other prior art vacuum interrupters utilize multiple contact systems in an axial configuration as illustrated in U.S. Pat. Nos. 6,255,615 B1, 6,720,515 B2 and patent application US 2008/0245772 A1. These vacuum interrupters engage one set of contacts by having the contact operating means move in one direction and engage a second set of contacts when the contact operating means moves in the opposite direction. This configuration is suitable for providing a means to ground the electric circuit in which the vacuum switch or interrupter is employed, but because the contact means is not capable of engaging both sets of contacts by moving in one direction, the vacuum interrupters do not provide a pre-insertion means.
Another prior art interrupter utilizes multiple contact systems wherein one set of contacts drives another as illustrated in U.S. Pat. No. 2,863,026. In this case the operating spring for the driven contact is mounted inside the interrupter and is subject to annealing during the brazing together of the interrupter. While work hardening will result in the return of some of the spring force characteristics, its final force characteristics will be uncontrolled. Additionally, this device is not suitable as a pre-insertion device as no means is provided to precisely position the driven contact or to adjust out the tolerance accumulation between the multiple parts.
While the aforementioned prior art arrangements may be suitable for their intended use in accordance with their respective defined applications, as discussed hereinbefore, it would be desirable to provide an efficient and compact pre-insertion contact arrangement contained within a vacuum switch module to limit transient in-rush currents and voltage transients.